Magnetic resonance imaging (“MRI”) offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, MRI can obtain images of soft tissues within the body which are not readily visualized using other imaging techniques. In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the nuclei to “precess” in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Tissues having a high density of nuclei will produce stronger signals than tissues with a low density of such nuclei. Furthermore, relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process, so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
Conventionally, MRI machines require that a patient lie in a horizontal position and then be advanced into a tubular enclosure within a super-conducting solenoidal magnet used to generate the static magnetic field. Ferromagnetic frame magnets having horizontal pole axes have been developed, which allow a patient to be imaged in a variety of positions including, for example, upright (sitting or standing), recumbent, Trendelenburg and reverse-Trendelenburg positions.
More specifically, ferromagnetic frame magnets having horizontal pole axes have been disclosed, for example, in commonly assigned U.S. Pat. No. 6,414,490, the disclosures of which are incorporated by reference herein, and U.S. Pat. No. 6,677,753, filed on Nov. 22, 2000, the disclosure of which is also incorporated by reference herein, a magnet having poles spaced apart from one another along a horizontal axis provides a horizontally oriented magnetic field within a patient-receiving gap between the poles. Such a magnet can be used with a patient positioning device including elevation and tilt mechanisms to provide extraordinary versatility in patient positioning. For example, where the patient positioning device includes a bed or similar device for supporting the patient in a supine or recumbent position, the bed can be tilted and/or elevated so as to image the patient in essentially any position between a fully standing position and a fully supine or fully recumbent position, and can be elevated or lowered so that essentially any portion of the patient's anatomy is disposed within the gap in an optimum position for imaging. As further disclosed in the aforesaid patents, the patient positioning device may include additional elements such as a platform, any type of seat, or both, projecting from the bed to support the patient when the bed is tilted towards a standing orientation. Still, other patient supporting devices can be used in place of a bed in a system of this type. Thus, magnets of this type provide extraordinary versatility in imaging.
For example, these systems allow evaluation of the spine in its all of its weight bearing (e.g., neutral, flexion and extension) and recumbent positions. In order to enable proper diagnosis of back pain, for example, it is usually important that a particular area of anatomical interest (e.g., a particular vertebrae) be evaluated in these different positions.
However, in switching from one of these positions to another, the patient is intentionally repositioned in the new position, resulting in a fairly large movement of the anatomy of interest both relative to neighboring anatomy, and relative to the imaging region in the magnet. Furthermore, repositioning a patient from one of the upright positions to the recumbent position, and vice versa, may involve removal of the patient from the imaging volume, and either removal or addition of a seat to the patient positioning bed before repositioning the patient in the imaging volume. Here certainly, the change in the position of the patient's anatomy is rather extreme. These examples illustrate the need to have a method and system, which would help insure alignment of imaging slices from scan to scan in different body positions.